Resonant Scattering of Light Atoms - Measuring Methods and Applications

Grochowski, J.; Serda, Paweł

doi:10.12693/aphyspola.82.147

Cited by 5 publications

(3 citation statements)

References 20 publications

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“…ω = 0, bisecting position) was developed in order to avoid accidental results of Bijvoet inequalities caused by random errors and to avoid azimuthal scan range where systematic errors could make the comparison unreliable. Recording of azimuthal scan for the determination of absolute structure is carried out for selected Friedel pairs in order to detect significant differences of two curves [22], avoiding regions of azimuthal angles, where systematic errors are particularly severe.…”

Section: Absolute Structure Determinationmentioning

confidence: 99%

Optimization of Azimuthal Scan Procedure for Absolute Structure Determination

Grochowski

Serda

1997

Acta Phys. Pol. A

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The paper describes the application of the azimuthal scan technique for absolute structure determination using X-ray anomalous dispersion. A diffraction pattern with rotation around the scattering vector provides information about various anisotropic effects: absorption, extinction and multi--beam interaction. Recording of azimuthal scans for Friedel pairs is proposed as an alternative to single reflection measurement. Several factors affecting the smoothness of the azimuthal scan curves are discussed and a procedure for eliminating various sources of errors, involving a Fourier filtering, is proposed in order to increase feasibility of chiral discrimination.

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Section: Absolute Structure Determinationmentioning

confidence: 99%

Optimization of Azimuthal Scan Procedure for Absolute Structure Determination

Grochowski

Serda

1997

Acta Phys. Pol. A

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“…(2) Employing a dedicated azimuthal scan method [12] which uses information carried out by azimuthal scan curve instead of a single number -reflection intensity in two-beam case. Azimuthal scan (ψ-scan) consists of a rotation of the crystal sample around the diffraction vector and recording of the resulting diffracted intensity variation as a function of the rotation angle 0.…”

Section: X-ray Anomalous Scatteringmentioning

confidence: 99%

“…The first deduction of the absolute configuration of organic molecule (NaRb tartrate) using X-ray anomalous scattering was carried out by Bijvoet [7] in 1951, and since then, the method is under constant development reaching the investigation of anomalous scattering anisotropy for heavy atoms [8] and in the area of organic and macromolecular crystals to make a practical use of anomalous scattering from light and medium-weight atoms for solving phase problems and chiral discrimination [9][10][11][12]. A significant part of the experimental progress was achieved owing to tunability of synchrotron radiation (SR) beams.…”

Section: Introductionmentioning

confidence: 99%

Absolute Structure Determination in Absence of Heavy Scatterers

Grochowski

1997

Acta Phys. Pol. A

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Feasibility of chiral discrimination using X‐ray anomalous scattering

Grochowski

Serda

1993

Chirality

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The paper discusses several feasibility prediction criteria with particular emphasis on light atom molecules. For several examples of refined absolute structures, feasibility predictions are calculated and compared with a posteriori calculated values of the Bijvoet ratio. An extension of the Pathasarathy table of the expected value of the Bijvoet ratio is given for parameter ranges corresponding to light anomalous scatterers. Practical conclusions are drawn for absolute structure determination of compounds containing oxygen or nitrogen as anomalous scatterers. o 1993 Wiey-Liss, Inc.KEY WORDS: anomalous dispersion, absolute structure, enantiomer determinationThe use of X-ray anomalous scattering for determination of absolute structures of crystalline substances and deduction of the absolute configuration of molecules was introduced by Coster and Knol in 1930' and was applied to organic substances by Bijvoet in 1949.' The method is being constantly developed, parallel with the improvement of diffraction techniques and registration of X-rays, especially since the advent of synchrotron radiation facilities.For organic chemists and pharmacologists, who operate within the range of small to medium size organic molecules, two questions are crucial: the feasibility of chiral discrimination and the reliability of enantiomer identification. The problem is a routine matter for small molecules containing relatively heavy atoms (Cl, S, Al). For such atoms, the anomalous dispersion component is relatively high, and the resulting anomalous dispersion contribution is easily observable.For lighter heteroatoms like F and 0, the feasibility of enantiomer determination is strongly dependent on several factors: atomic fraction of the "heavy atom," structural parameters describing chirality, and, on the experimental side, the choice of proper radiation and strategy of measurements. Basic DefinitionsDispersion corrections to scattering factors Atomic scattering factors fo for X-rays are calculated and tabulated on the assumption that the frequency of the incident radiation is large compared with any absorption frequency of the atom; a further assumption is also made that all electrons scatter as free classical electrons with interference due to spatial distribution. The scattering power of a bound electron as well as the phase of the scattered X-rays may be different from those for a free electron. The atomic scattering power may be written in the form:where Af' and Af' denote the real and imaginary dispersion corrections. We define the anomalous phase rotation by k = Af t ' @' + Af ') and so equation (1) can be cast in the form:The values of the dispersion corrections depend upon the distance on the energy scale between the incident radiation and the absorption edge of the scattering atom. Dispersion corrections are less sensitive a function of the diffraction angle than f", since the dispersion effect appears predominantly in the inner shells of the atom. A comprehensive list of the dispersion corrections is published in the I...

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Resonant Scattering of Light Atoms - Measuring Methods and Applications

Cited by 5 publications

References 20 publications

Optimization of Azimuthal Scan Procedure for Absolute Structure Determination

Optimization of Azimuthal Scan Procedure for Absolute Structure Determination

Absolute Structure Determination in Absence of Heavy Scatterers

Feasibility of chiral discrimination using X‐ray anomalous scattering

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